Method for preparing surface-modified semiconductive and metallic nanoparticles having enhanced dispersibility in aqueous media

ABSTRACT

Water-dispersible nanoparticles are prepared by applying a coating of a multiply amphipathic dispersant to the surface of a hydrophobic nanoparticle comprised of a semiconductive or metallic material. The multiply amphipathic dispersant has two or more hydrophobic regions and two or more hydrophilic regions, and is typically polymeric. Preferred polymeric dispersants are comprised of (1) a hydrophobic backbone with hydrophilic branches, (2) a hydrophilic backbone with hydrophobic branches, or (3) a backbone that may be either hydrophobic or hydrophilic, and substituted with both hydrophilic and hydrophobic branches. Monodisperse populations of water-dispersible nanoparticles are also provided, as are conjugates of the water-dispersible nanoparticles with affinity molecules such as peptides, oligonucleotides, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/841,237, filed Apr. 23, 2001, which claims priority to U.S.Provisional Application No. 60/240,216, filed Oct. 13, 2000. Thedisclosures of the aforementioned applications are incorporated byreference in their entireties.

TECHNICAL FIELD

This invention relates generally to surface-modified nanoparticles, andmore particularly relates to surface-modified semiconductor and metalnanoparticles having enhanced dispersibility in aqueous media as well assuperior colloidal and photophysical stability. The inventionadditionally relates to methods for making and using the novelsurface-modified nanoparticles. The invention finds utility in a varietyof fields, including biology, analytical and combinatorial chemistry,medical diagnostics, and genetic analysis.

BACKGROUND

Semiconductor nanocrystals (also known as quantum dot particles) whoseradii are smaller than the bulk exciton Bohr radius constitute a classof materials intermediate between molecular and bulk forms of matter.Quantum confinement of both the electron and hole in all threedimensions leads to an increase in the effective band gap of thematerial with decreasing crystallite size. Consequently, both theoptical absorption and emission of semiconductor nanocrystals shift tothe blue (higher energies) as the size of the nanocrystals gets smaller.

Semiconductor nanocrystals are nanoparticles composed of an inorganic,crystalline semiconductive material and have unique photophysical,photochemical and nonlinear optical properties arising from quantum sizeeffects, and have therefore attracted a great deal of attention fortheir potential applicability in a variety of contexts, e.g., asdetectable labels in biological applications, and as useful materials inthe areas of photocatalysis, charge transfer devices, and analyticalchemistry. As a result of the increasing interest in semiconductornanocrystals, there is now a fairly substantial body of literaturepertaining to methods for manufacturing such nanocrystals. Broadly,these routes may be classified as involving preparation in glasses (seeEkimov et al. (1981) JETP Letters 34:345), aqueous preparation(including preparation that involve use of inverse micelles, zeolites,Langmuir-Blodgett films, and chelating polymers; see Fendler et al.(1984) J. Chem. Society, Chemical Communications 90:90, and Henglein etal. (1984) Ber. Bunsenges. Phys. Chem. 88:969), and high temperaturepyrolysis of organometallic semiconductor precursor materials (Murray etal. (1993) J. Am. Chem. Soc. 115:8706; Katari et al. (1994) J. Phys.Chem. 98:4109). The two former methods yield particles that haveunacceptably low quantum yields for most applications, a high degree ofpolydispersity, poor colloidal stability, a high degree of internaldefects, and poorly passivated surface trap sites. In addition,nanocrystals made by the first route are physically confined to a glassmatrix and cannot be further processed after synthesis.

To date, only the high temperature pyrolysis of organometallic reagentshas yielded semiconductor nanocrystals that are internally defect free,possess high band edge luminescence and no trapped emission, and exhibitnear monodispersity. Additionally, this route gives the syntheticchemist a substantial degree of control over the size of the particlesprepared. See Murray et al. (1993), supra. One disadvantage of thismethod, however, is that the particles are sequestered in reversemicelles of coordinated, hydrophobic surfactant molecules. As such, theyare only dispersible in organic solvents such as chloroform,dichloromethane, hexane, toluene, and pyridine. This is problematicinsofar as many applications that rely on the fluorescence emission ofthe semiconductor nanocrystals require that the nanocrystals be watersoluble or at least water dispersible.

Although some methods for rendering semiconductor nanocrystals waterdispersible have been reported, they are still problematic insofar asthe treated semiconductor nanocrystals suffer from significantdisadvantages that limit their wide applicability. For example, Spanhelet al. (1987) J. Am. Chem. Soc. 109:5649, discloses a Cd(OH)₂-capped CdSsol; however, the photoluminescent properties of the sol were pHdependent. The sol could be prepared only in a very narrow pH range (pH8-10) and exhibited a narrow fluorescence band only at a pH of greaterthan 10. Such pH dependency greatly limits the usefulness of thematerial; in particular, it is not appropriate for use in biologicalsystems.

Other groups have replaced the organic passivating layer of thesemiconductor nanocrystal with water-soluble moieties; however, theresultant derivatized semiconductor nanocrystals are not highlyluminescent. Short chain thiols such as 2-mercaptoethanol and1-thio-glycerol have been used as stabilizers in the preparation ofwater-soluble CdTe nanocrystals. See, Rogach et al. (1996) Ber.Bunsenges. Phys. Chem. 100:1772 and Rajh et al. (1993) J. Phys. Chem.97:11999. Other more exotic capping compounds have been reported withsimilar results. See Coffer et al. (1992) Nanotechnology 3:69, whichdescribes the use of deoxyribonucleic acid (DNA) as a capping compound.In all of these systems, the coated semiconductor nanocrystals were notstable and photoluminescent properties degraded with time.

Thus, to use these high quantum yield materials in applications thatrequire an aqueous medium, one must find a way of changing the polarityof the organic coating, thereby facilitating the transfer of theseparticles to water. A great deal of work has been conducted on surfaceexchange reactions that seek to replace the oleophilic hydrocarboncoating on the nanocrystal surface with a range of bifunctional polarmolecules wherein one functional group of the capping molecule bearssome affinity for the surface of the nanocrystal, while the otherfunctional group, by virtue of its ionizability or high degree ofhydration, renders the nanocrystal water soluble. For example,International Patent Publication No. WO 00/17655 to Bawendi et al.describes a method for rendering semiconductor nanocrystals waterdispersible wherein monomeric surfactants are used as dispersing agents,with the hydrophobic region of the surfactants promoting associationwith the nanocrystals, while the hydrophilic region has affinity for anaqueous medium and stabilizes an aqueous suspension of the nanocrystals.International Patent Publication No. WO 00/17656 to Bawendi et al.describes a similar method wherein monomeric compounds of formulaHS-(CH₂)_(n)-X, wherein n is preferably ≧10 and X is carboxylate orsulfonate, are used in place of the monomeric surfactants.

Kuno et al. (1997) J. Chem. Phys. 106:9869-9882, Mikulec, “SemiconductorNanocrystal Colloids: Manganese Doped Cadmium Selenide, (Core)ShellComposites for Biological Labeling, and Highly Fluorescent CadmiumTelluride,” doctoral dissertation, Massachusetts Institute of Technology(September 1999), and International Patent Publication No. WO 00/17656to Bawendi et al., cited supra, give detailed descriptions of surfaceexchange reactions designed to improve the water dispersibility ofhydrophobic nanocrystals. In general, these references indicate that:exchange of the original hydrophobic surfactant layer on the nanocrystalsurface is never quite complete, with retention of only about 10% toabout 15% of the surfactant (even after multiple exchange reactions);although never quantitatively displaced, exchange of the originalphosphine/phosphine oxide surfactant layer with more polar ligandsresults in a substantial decrease in quantum yield that is neverentirely regained; once⁻dispersed in water, the particles have limitedcolloidal stability; and attempts to carry out further chemistry withthese particles, such as linking them to biomolecules through theirpendant carboxyl functionalities, is highly irreproducible and dependenton the size of the nanocrystal.

Thus, there remains a need in the art for a reliable, reproduciblemethod for rendering hydrophobic semiconductor nanocrystals dispersiblein aqueous media while preserving the quantum efficiencies of theoriginal particles, maintaining colloidal stability, and avoiding orminimizing any change in particle size distribution. Ideally, such amethod would be useful not only with semiconductor nanoparticles, butalso with other types of nanoparticles having hydrophobic surfaces,e.g., semiconductive nanoparticles that are not necessarily crystallineand metallic nanoparticles that may or may not be surface-modified.

SUMMARY OF THE INVENTION

It is accordingly a primary object of the invention to address theaforementioned need in the art by providing surface-modifiednanoparticles having enhanced dispersibility in aqueous media, whereinthe nanoparticles are comprised of an inner core having a hydrophobicsurface and an outer layer of a multiply amphipathic dispersant.

It is still another object of the invention to provide suchsurface-modified nanoparticles wherein the inner core is composed of asemiconductive or metallic material.

It is yet another object of the invention to provide such nanoparticleswherein the multiply amphipathic dispersant is a polymer having two ormore hydrophobic regions and two or more hydrophilic regions.

It is a further object of the invention to provide a method forpreparing a population of the aforementioned water-dispersiblenanoparticles.

It is still a further object of the invention to provide a compositioncomposed of a nanoparticle conjugate, i.e., a water-dispersiblenanoparticle as above, conjugated to an affinity molecule that serves asthe first member of a binding pair.

It is yet a further object of the invention to provide such acomposition wherein a second member of the binding pair is associatedwith the first member through either covalent or noncovalentinteraction.

It is an additional object of the invention to provide a monodispersepopulation of water-dispersible nanoparticles wherein the population ischaracterized in that it exhibits no more than about a 10% rmsdeviation, preferably no more than about a 5% rms deviation, in thediameter of the inner core.

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned by practice of the invention.

In one aspect of the invention, then, a water-dispersible nanoparticleis provided that is comprised of an inner core and an outer layer of amultiply amphipathic dispersant, i.e., a compound having two or morehydrophobic regions and two or more hydrophilic regions. The inner corecomprises a semiconductive or metallic material, preferably an inorganicsemiconductive material that is in a crystalline state. Generally, theinner core also comprises a hydrophobic passivating layer on thesemiconductive or metallic material resulting from solvents and/orsurfactants used in nanoparticle manufacture. The surface of the innercore is accordingly hydrophobic, and the hydrophobic regions of thedispersant thus have affinity for the core surface and attach thereto,while the hydrophilic regions of the dispersant extend outward from thenanoparticle and provide for dispersibility in water. In a preferredembodiment, the dispersant is polymeric and has a plurality of bothhydrophobic regions and hydrophilic regions, thus enhancing waterdispersibility of the nanoparticle as well as the dispersant's affinityfor the core surface. Particularly preferred dispersants arehyperbranched or dendritic polymers, which, relative to prior methodsthat involve monomeric dispersants, substantially increase the waterdispersibility and colloidal stability of the nanoparticles. In apreferred embodiment, the nanoparticles are luminescent semiconductivenanocrystals, and include an overcoating “shell” layer between the innercore and the multiply amphipathic outer layer to increase luminescenceefficiency. The shell material has a higher bandgap energy than thenanocrystal core, and should also have good conduction and valence bandoffset with respect to the nanocrystal core. Further, an “affinitymolecule,” i.e., one member of a binding pair, may be attached to theouter layer of the surface-modified molecule, providing a nanoparticle“conjugate” that is useful in detecting the presence or quantity oftarget molecules that comprise the second member of the binding pair.The affinity molecule may be, for example, a protein, anoligonucleotide, an enzyme inhibitor, a polysaccharide, or a smallmolecule having a molecular weight of less than about 1500 grams/Mol.

In a related aspect of the invention, then, a composition is providedthat is comprised of the aforementioned nanoparticle conjugate inassociation with the second member of the binding pair, wherein theassociation may involve either covalent or noncovalent interaction.

In another aspect of the invention, a monodisperse population ofsurface-modified nanoparticles is provided, comprising a plurality ofwater-dispersible nanoparticles each having an inner core comprised of asemiconductive or metallic material and, surrounding the inner core, anouter layer comprised of a multiply amphipathic dispersant as describedabove, wherein the population is characterized in that the nanoparticlesare of substantially the same size and shape, i.e., the populationexhibits no more than about a 10% rms deviation in the diameter of theinner core, preferably no more than about a 5% rms deviation in thediameter of the inner core. The narrow size distribution of amonodisperse population increases the “information density” that isobtainable as a result of the particles' luminescence, i.e., the numberof discrete luminescence emissions obtainable for a given nanoparticlecomposition.

In another aspect of the invention, a method is provided for making thesurface-modified nanoparticles described above. The method involves (a)admixing (i) an amphipathic dispersant comprised of a polymer having twoor more hydrophobic regions and two or more hydrophilic regions, with(ii) a plurality of hydrophobic nanoparticles, in (iii) a nonaqueoussolvent, to provide an admixture of dispersant and nanoparticles insolution; (b) subjecting the admixture to conditions effective to causeadsorption of the dispersant by the nanoparticles; and (c) transferringthe dispersant-coated nanoparticles prepared in step (b) to an aqueousmedium such as water or an aqueous buffer.

DETAILED DESCRIPTION OF THE INVENTION I. Definitions:

Before describing the present invention in detail, it is to beunderstood that unless otherwise indicated this invention is not limitedto specific nanoparticle materials, amphipathic dispersants, ormanufacturing processes, as such may vary. It is also to be understoodthat the terminology used herein is for the purpose of describingparticular embodiments only, and is not intended to be limiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “an” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example, “adispersant” refers to a single dispersant as well as a mixture of two ormore dispersants, “a nanoparticle” encompasses not only a singlenanoparticle but also two or more nanoparticles, and the like.

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow,

The term “amphipathic,” referring to the dispersants employed herein, isused in its conventional sense to indicate a molecular species having ahydrophobic region and a hydrophilic region. The dispersants herein are“multiply amphipathic” in that they contain two or more hydrophobicregions and two or more hydrophilic regions.

The term “attached,” as in, for example, the “attachment” of adispersant to a nanoparticle surface, includes covalent binding,adsorption, and physical immobilization. The terms “associated with,”“binding” and “bound” are identical in meaning to the term “attached.”

Attachment of the present multiply amphipathic dispersants to thesurface of a metallic or semiconductive nanoparticle will generallyinvolve “adsorption,” wherein “adsorption” refers to the noncovalentretention of a molecule by a substrate surface. That is, adsorptionoccurs as a result of noncovalent interaction between a substratesurface and adsorbing moieties present on the molecule that is adsorbed.Adsorption may occur through hydrogen bonding, van der Waal's forces,polar attraction or electrostatic forces (i.e., through ionic bonding),and in the present case will typically involve the natural affinity of ahydrophobic region of a molecule for a hydrophobic surface.

The term “nanoparticle” refers to a particle, generally a semiconductiveor metallic particle, having a diameter in the range of about 1 nm toabout 1000 nm, preferably in the range of about 2 nm to about 50 nm,more preferably in the range of about 2 nm to about 20 nm. As discussedelsewhere herein, semiconductive, and metallic “nanoparticles” generallyinclude a passivating layer of a water-insoluble organic material thatresults from the method used to manufacture such nanoparticles. Theterms “surface-modified nanoparticle” and “water-dispersiblenanoparticle” as used herein refer to the modified nanoparticles of theinvention, while the term “nanoparticle,” without qualification, refersto the hydrophobic nanoparticle that serves as the inner core of thesurface-modified, water-dispersible nanoparticle.

The terms “semiconductor nanoparticle” and “semiconductive nanoparticle”refer to a nanoparticle as defined above that is composed of aninorganic semiconductive material, an alloy or other mixture ofinorganic semiconductive materials, an organic semiconductive material,or an inorganic or organic semiconductive core contained within one ormore semiconductive overcoat layers.

The term “metallic nanoparticle” refers to a nanoparticle as definedabove that is composed of a metallic material, an alloy or other mixtureof metallic materials, or a metallic core contained within one or moremetallic overcoat layers.

The terms “semiconductor nanocrystal,” “quantum dot” and “Qdot®nanocrystal” are used interchangeably herein to refer to semiconductornanoparticles composed of an inorganic crystalline material that isluminescent (i.e., they are capable of emitting electromagneticradiation upon excitation), and include an inner core of one or morefirst semiconductor materials that is optionally contained within anovercoating or “shell” of a second semiconductor material. Asemiconductor nanocrystal core surrounded by a semiconductor shell isreferred to as a “core/shell” semiconductor nanocrystal. The surroundingshell material will preferably have a bandgap energy that is larger thanthe bandgap energy of the core material and may be chosen to have anatomic spacing close to that of the core substrate. Suitablesemiconductor materials for the core and/or shell include, but are notlimited to, the following: materials comprised of a first elementselected from Groups 2 and 12 of the Periodic Table of the Elements anda second element selected from Group 16 (e.g., ZnS, ZnSe, ZnTe, CDs,CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS,SrSe, SrTe, BaS, BaSe, BaTe, and the like); materials comprised of afirst element selected from Group 13 of the Periodic Table of theElements and a second element selected from Group 15 (GaN, GaP, GaAs,GaSb, InN, InP, InAs, InSb, and the like); materials comprised of aGroup 14 element (Ge, Si, and the like); materials such as PbS, PbSe andthe like; and alloys and mixtures thereof. As used herein, all referenceto the Periodic Table of the Elements and groups thereof is to the newIUPAC system for numbering element groups, as set forth in the Handbookof Chemistry and Physics, 81^(st) Edition (CRC Press, 2000).

By “luminescence” is meant the process of emitting electromagneticradiation (light) from an object. Luminescence results when a systemundergoes a transition from an excited state to a lower energy statewith a corresponding release of energy in the form of a photon. Theseenergy states can be electronic, vibrational, rotational, or anycombination thereof. The transition responsible for luminescence can bestimulated through the release of energy stored in the system chemicallyor added to the system from an external source. The external source ofenergy can be of a variety of types including chemical, thermal,electrical, magnetic, electromagnetic, and physical, or any other typeof energy source capable of causing a system to be excited into a statehigher in energy than the ground state. For example, a system can beexcited by absorbing a photon of light, by being placed in an electricalfield, or through a chemical oxidation-reduction reaction. The energy ofthe photons emitted during luminescence can be in a range fromlow-energy microwave radiation to high-energy x-ray radiation.Typically, luminescence refers to photons in the range from UV to IRradiation.

The term “monodisperse” refers to a population of particles (e.g., acolloidal system) wherein the particles have substantially identicalsize and shape. For the purpose of the present invention, a“monodisperse” population of particles means that at least about 60% ofthe particles, preferably about 75% to about 90% of the particles, fallwithin a specified particle size range. A population of monodisperseparticles deviates less than 10% rms (root-mean-square) in diameter andpreferably less than 5% rms.

The phrase “one or more sizes of nanoparticles” is used synonymouslywith the phrase “one or more particle size distributions ofnanoparticles.” One of ordinary skill in the art will realize thatparticular sizes of nanoparticles such as semiconductor nanocrystals areactually obtained as particle size distributions.

By use of the term “narrow wavelength band” or “narrow spectrallinewidth” with regard to the electromagnetic radiation emission of thesemiconductor nanocrystal is meant a wavelength band of emissions notexceeding about 60 nm, and preferably not exceeding about 30 nm inwidth, more preferably not exceeding about 20 nm in width, and symmetricabout the center. It should be noted that the bandwidths referred to aredetermined from measurement of the full width of the emissions at halfpeak height (FWHM), and are appropriate in the range of 200 nm to 2000nm.

By use of the term “a broad wavelength band,” with regard to theexcitation of the semiconductor nanocrystal is meant absorption ofradiation having a wavelength equal to, or shorter than, the wavelengthof the onset radiation (the onset radiation is understood to be thelongest wavelength (lowest energy) radiation capable of being absorbedby the semiconductor nanocrystal). This onset occurs near to, but atslightly higher energy than the “narrow wavelength band” of theemission. This is in contrast to the “narrow absorption band” of dyemolecules, which occurs near the emission peak on the high energy side,but drops off rapidly away from that wavelength and is often negligibleat wavelengths further than 100 nm from the emission.

The term “emission peak” refers to the wavelength of light within thecharacteristic emission spectra exhibited by a particular semiconductornanocrystal size distribution that demonstrates the highest relativeintensity.

The term “excitation wavelength” refers to light having a wavelengthlower than the emission peak of the semiconductor nanocrystal used inthe first detection reagent.

A “hydrophobic” compound (e.g., a “hydrophobic” monomer) is one thatwill transfer from an aqueous phase to an organic phase, specificallyfrom water to an organic, water-immiscible nonpolar solvent with adielectric constant ≦5, with a partition coefficient of greater thanabout 50%. A “hydrophobic monomer unit” refers to a hydrophobic monomeras it exists within a polymer. A “hydrophobic region” refers to ahydrophobic molecular segment, e.g., a molecular segment within apolymer. A “hydrophobic region” may be a single hydrophobic monomer unitor two or more hydrophobic monomer units that may be the same ordifferent and may or may not be adjacent.

A “hydrophilic” compound (e.g., a “hydrophilic” monomer) is one thatwill transfer from an organic phase to an aqueous phase, specificallyfrom an organic, water-immiscible nonpolar solvent with a dielectricconstant <5 to water, with a partition coefficient of greater than about50%. A “hydrophilic monomer unit” refers to a hydrophilic monomer as itexists in a polymeric segment or polymer. A “hydrophilic region” refersto a hydrophilic molecular segment, e.g., a hydrophilic molecularsegment within a polymer. A “hydrophilic region” may be a singlehydrophilic monomer unit or two or more hydrophilic monomer units thatmay be the same or different and may or may not be adjacent.

The term “ionizable” refers to a group that is electronically neutral ata specific pH, but can be ionized and thus rendered positively ornegatively charged at higher or lower pH, respectively.

The term “alkyl” as used herein refers to a branched or unbranchedsaturated hydrocarbon group of 1 to approximately 24 carbon atoms, suchas methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl,octyl, decyl, tetradecyl, hexadecyl, eicosyl and tetracosyl, as well ascycloalkyl groups such as cyclopentyl and cyclohexyl. The term “loweralkyl” intends an alkyl group of 1 to 4 carbon atoms, and thus includesmethyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and t-butyl.

The term “alkylene” as used herein refers to a difunctional saturatedbranched or unbranched hydrocarbon chain containing from 1 toapproximately 24 carbon atoms, typically 1 to approximately 12 carbonatoms, and includes, for example, methylene (—CH₂—), ethylene(—CH₂—CH₂—), propylene (—CH₂—CH₂—CH₂—), 2-methylpropylene(—CH₂—CH(CH₃)—CH₂—), hexylene (—(CH₂)₆—), and the like. “Loweralkylene,” as in the lower alkylene linkage of the optional couplingagent herein, refers to an alkylene group of 1 to 4 carbon atoms.

The term “alkenyl” as used herein refers to a branched or unbranchedhydrocarbon group typically although not necessarily containing 2 toabout 24 carbon atoms and at least one double bond, such as ethenyl,n-propenyl, isopropenyl, n-butenyl, isobutenyl, octenyl, decenyl, andthe like. Generally, although not necessarily, alkenyl groups hereincontain 2 to about 12 carbon atoms. The term “lower alkenyl” intends analkenyl group of 2 to 4 carbon atoms, and the term “alkenylene” refersto a difunctional alkenyl group, in the same way that the term“alkylene” refers to a difunctional alkyl group.

The term “alkynyl” as used herein refers to a branched or unbranchedhydrocarbon group typically although not necessarily containing 2 toabout 24 carbon atoms and at least one triple bond, such as ethynyl,n-propynyl, isopropynyl, n-butynyl, isobutynyl, octynyl, decynyl, andthe like. Generally, although again not necessarily, alkynyl groupsherein contain 2 to about 12 carbon atoms. The term “lower alkynyl”intends an alkynyl group of 2 to 4 carbon atoms, preferably 3 or 4carbon atoms.

The term “heteroatom-containing” and the prefix “hetero-,” as in“heteroatom-containing alkyl” and “heteroalkyl,” refer to a molecule ormolecular fragment in which one or more carbon atoms is replaced with anatom other carbon, e.g., nitrogen, oxygen, sulfur, phosphorus orsilicon.

The term “alkoxy” as used herein refers to a substituent —O—R wherein Ris alkyl as defined above. The term “lower alkoxy” refers to such agroup wherein R is lower alkyl as defined above, e.g., methoxy, ethoxyand the like. The term “aryl” as used herein, and unless otherwisespecified, refers to an aromatic moiety containing 1 to 3 aromaticrings. For aryl groups containing more than one aromatic ring, the ringsmay be fused or linked. Aryl groups are optionally substituted with oneor more inert, nonhydrogen substituents per ring; suitable “inert,nonhydrogen” substituents include, for example, halo, haloalkyl(preferably halo-substituted lower alkyl), alkyl (preferably loweralkyl), alkenyl (preferably lower alkenyl), alkynyl (preferably loweralkynyl), alkoxy (preferably lower alkoxy), alkoxycarbonyl (preferablylower alkoxycarbonyl), carboxy, nitro, cyano and sulfonyl. Unlessotherwise indicated, the term “aryl” is also intended to includeheteroaromatic moieties, i.e., aromatic heterocycles. Generally,although not necessarily, the heteroatoms will be nitrogen, oxygen orsulfur. The term “arylene” refers to a difunctional aryl moiety in thesame way that the term “alkylene” refers to a difunctional alkyl group.

The term “aralkyl” refers to an alkyl group with an aryl substituent,and the term “aralkylene” refers to an alkylene group with an arylsubstituent; the term “alkaryl” refers to an aryl group that has analkyl substituent, and the term “alkarylene” refers to an arylene groupwith an alkyl substituent.

The terms “halo” and “halogen” are used in the conventional sense torefer to a chloro, bromo, fluoro, or iodo substituent. The term“haloalkyl” refers to an alkyl group in which at least one of thehydrogen atoms in the group has been replaced with a halogen atom.

The term “peptide” refers to oligomers or polymers of any length whereinthe constituent monomers are alpha amino acids linked through amidebonds, and encompasses amino acid dimers as well as polypeptides,peptide fragments, peptide analogs, naturally occurring proteins,mutated, variant, or chemically modified proteins, fusion proteins, andthe like. The amino acids of the peptide molecules may be any of thetwenty conventional amino acids, stereoisomers (e.g., D-amino acids) ofthe conventional amino acids, structural variants of the conventionalamino acids, e.g., iso-valine, or non-naturally occurring amino acidssuch as α,α-disubstituted amino acids, N-alkyl amino acids, β-alanine,naphthylalanine, 3-pyridylalanine, 4-hydroxyproline, 0-phosphoserine,N-acetylserine, N-formylmethionine, 3-methylhistidine, 5-hydroxylysine,and nor-leucine. In addition, the term “peptide” encompasses peptideswith posttranslational modifications such as glycosylations,acetylations, phosphorylations, and the like. The term “oligonucleotide”is used herein to include a polymeric form of nucleotides of any length,either ribonucleotides or deoxyribonucleotides. This term refers only tothe primary structure of the molecule. Thus, the term includes triple-,double- and single-stranded DNA, as well as triple-, double- andsingle-stranded RNA. It also includes modifications, such as bymethylation and/or by capping, and unmodified forms of theoligonucleotide. More particularly, the term includespolydeoxyribonucleotides (containing 2-deoxy-D-ribose),polyribonucleotides (containing D-ribose), any other type ofpolynucleotide which is an N- or C-glycoside of a purine or pyrimidinebase, and other polymers containing nonnucleotidic backbones, forexample, polyamide (e.g., peptide nucleic acids (PNAs)) andpolymorpholino (commercially available from the Anti-Virals, Inc.,Corvallis, Oreg., as Neugene) polymers, and other syntheticsequence-specific nucleic acid polymers, providing that the polymerscontain nucleobases in a configuration that allows for base pairing andbase stacking, such as is found in DNA and RNA. There is no intendeddistinction in length between the terms “polynucleotide,”“oligonucleotide,” “nucleic acid” and “nucleic acid molecule,” and theseterms refer only to the primary structure of the molecule. Thus, theseterms include, for example, 3′-deoxy-2′,5′-DNA, oligodeoxyribonucleotideN3′ P5′ phosphoramidates, 2′-O-alkyl-substituted RNA, double- andsingle-stranded DNA, as well as double- and single-stranded RNA, DNA:RNAhybrids, and hybrids between PNAs and DNA or RNA, and also include knowntypes of modifications, for example, labels which are known in the art,methylation, “caps,” substitution of one or more of the naturallyoccurring nucleotides with an analog, intemucleotide modifications suchas, for, example, those with uncharged linkages (e.g., methylphosphonates, phosphotriesters, phosphoramidates, carbamates, etc.),with negatively charged linkages (e.g., phosphorothioates,phosphorodithioates, etc.), and with positively charged linkages (e.g.,aminoalklyphosphoramidates, aminoalkylphosphotriesters), thosecontaining pendant moieties, such as, for example, proteins (includingnucleases, toxins, antibodies, signal peptides, poly-L-lysine, etc.),those with intercalators (e.g., acridine, psoralen, etc.), thosecontaining chelators (e.g., metals, radioactive metals, boron, oxidativemetals, etc.), those containing alkylators, those with modified linkages(e.g., alpha anomeric nucleic acids, etc.), as well as unmodified formsof the polynucleotide or oligonucleotide.

The term “polymer” is used herein in its conventional sense to refer toa compound having two or more monomer units, and is intended to includelinear and branched polymers, the term “branched polymers” encompassingsimple branched structures as well as hyperbranched and dendriticpolymers. The term “monomer” is used herein to refer to compounds thatare not polymeric. “Polymers” herein may be naturally occurring,chemically modified, or chemically synthesized.

The term “water-dispersible” as used herein refers to an essentiallyunaggregated dispersion of particles, such that discrete particles ofapproximately 2 nm to 50 nm can be sustained indefinitely at highconcentrations (10-20 μM).

The term “binding pair” refers to first and second molecules thatspecifically bind to each other. “Specific binding” of the first memberof the binding pair to the second member of the binding pair in a sampleis evidenced by the binding of the first member to the second member, orvice versa, with greater affinity and specificity than to othercomponents in the sample. The binding between the members of the bindingpair is typically noncovalent. The terms “affinity molecule” and “targetanalyte” are also used herein to refer to the first and second membersof a binding pair, respectively. Exemplary binding pairs include anyhaptenic or antigenic compound in combination with a correspondingantibody or binding portion or fragment thereof (e.g., digoxigenin andanti-digoxigenin; mouse immunoglobulin and goat anti-mouseimmunoglobulin) and nonimmunological binding pairs (e.g., biotin-avidin,biotin-streptavidin, hormone [e.g., thyroxine and cortisol]-hormonebinding protein, receptor-receptor agonist or antagonist (e.g.,acetylcholine receptor-acetylcholine or an analog thereof), IgG-proteinA, lectin-carbohydrate, enzyme-enzyme cofactor, enzyme-enzyme inhibitor,and complementary polynucleotide pairs capable of forming nucleic acidduplexes), and the like.

A “nanoparticle conjugate” refers to a nanoparticle linked, through anouter layer of an amphipathic dispersant, to a member of a “bindingpair” that will selectively bind to a detectable substance present in asample, e.g., a biological sample. The first member of the binding pairlinked to the nanoparticle can comprise any molecule, or portion of anymolecule, that is capable of being linked to the nanoparticle and that,when so linked, is capable of specifically recognizing the second memberof the binding pair.

All molecular weights specified herein are number average molecularweights.

H. The Nanoparticles:

Prior to surface modification with a multiply amphipathic dispersant,the nanoparticles of the invention are nanoparticles with hydrophobicsurfaces, the particles having a diameter in the range of about 1 nm toabout 1000 nm, preferably in the range of about 2 rim to about 50 nm,more preferably in the range of about 2 nm to about 20 nm. Generally,the nanoparticles will be comprised of a semiconductive or metallicmaterial, with semiconductive nanoparticles preferred. Also, as will beexplained in greater detail below, the semiconductive or metallicmaterial typically has a coating of a hydrophobic passivating layerresulting from the use of solvents and/or surfactants duringnanoparticle manufacture. The hydrophobic surfaces of the nanoparticleshave affinity for and thus serve to attach the amphipathic dispersant byvirtue of the hydrophobic regions within the dispersant.

Semiconductive nanoparticles may be composed of an organic semiconductormaterial or an inorganic semiconductor material. Organic semiconductormaterials will generally be conjugated polymers. Suitable conjugatedpolymers include, for example, cis and trans polyacetylenes,polydiacetylenes, polyparaphenylenes, polypyrroles, polythiophenes,polybithiophenes, polyisothianaphthene, polythienylvinylenes,polyphenylenesulfide, polyaniline, polyphenylenevinylenes, andpolyphenylenevinylene derivatives, e.g.,poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene (“MEH-PPV”)(see U.S. Pat. No. 5,189,136 to Wudl et al.), poly(2,5-bischelostanoxy-1,4-phenylene vinylene) (“BCHA-PPV”) (e.g., asdescribed in International Patent Publication No. WO 98/27136), andpoly(2-N,N-dimethylamino phenylene vinylene)(described in U.S. Pat. No.5,604,292 to Stenger-Smith et al.). Inorganic semiconductivenanoparticles are, however, preferred, and are optimally crystalline innature; such nanoparticles are termed “semiconductor nanocrystals”herein. Semiconductor nanocrystals are capable of luminescence,generally fluorescence, when excited by light. Currently, detection ofbiological compounds by photoluminescence utilizes fluorescent organicdyes and chemiluminescent compounds. The use of semiconductornanocrystals as luminescent markers, particularly in biological systems,provides advantages over existing fluorescent dyes. Many of theseadvantages relate to the spectral properties of nanocrystals, e.g., theability to control the composition and size of nanocrystals enables oneto construct nanocrystals with fluorescent emissions at any wavelengthin the UV-visible-IR regions. With respect to composition, for example,semiconductor nanocrystals that emit energy in the visible rangeinclude, but are not limited to, CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, andGaAs. Semiconductor nanocrystals that emit energy in the near IR rangeinclude, but are not limited to, InP, InAs, InSb, PbS, and PbSe.Finally, semiconductor nanocrystals that emit energy in the blue tonear-ultraviolet include, but are not limited to, ZnS and GaN. For anyparticular nanocrystal composition, it is also possible to tune theemission to a desired wavelength by controlling particle sizedistribution. In preferred embodiments, 5-20 discrete emissions (five totwenty different size populations or distributions distinguishable fromone another), more preferably 10-15 discrete emissions, are obtained forany particular composition, although one of ordinary skill in the artwill realize that fewer than five emissions and more than twentyemissions could be obtained depending on the monodispersity of thesemiconductor nanocrystal particle population. If high informationdensity is required, and thus a greater number of distinct emissions,the nanocrystals are preferably substantiall_(y) monodisperse within thesize range given above.

As explained above, “monodisperse” refers to a population of particles(e.g., a colloidal system) in which the particles have substantiallyidentical size and shape. In preferred embodiments for high informationdensity applications, monodisperse particles deviate less than 10% rmsin diameter, and preferably less than 5% rms. Monodisperse semiconductornanocrystals have been described in detail in Murray et al. (1993) J.Am. Chem. Soc. 115:8706, and in Murray, “Synthesis and Characterizationof II-VI Quantum Dots and Their Assembly into 3-D Quantum DotSuperlattices,” doctoral dissertation, Massachusetts Institute ofTechnology (1995). One of ordinary skill in the art will also realizethat the number of discrete emissions that can be distinctly observedfor a given composition depends not only upon the monodispersity of theparticles, but also on the deconvolution techniques employed.Semiconductor nanocrystals, unlike dye molecules, can be easily modeledas Gaussians and therefore are more easily and more accuratelydeconvoluted.

However, for some applications, high information density will not berequired and it may be more economically attractive to use morepolydisperse particles. Thus, for applications that do not require highinformation density, the linewidth of the emission may be in the rangeof 40-60 nm.

Semiconductor nanocrystals may be made using techniques known in theart. See, e.g., U.S. Pat. Nos. 6,048,616, 5,990,479, 5,690,807,5,505,928 and 5,262,357, as well as International Patent Publication No.WO 99/26299, published May 27, 1999. In particular, exemplary materialsfor use as semiconductor nanocrystals in the biological and chemicalassays of the present invention include, but are not limited to, thosedescribed above, including Group 2-16, 12-16, 13-15 and 14semiconductors such as ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, MgS, MgSe,MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, GaN, GaP, GaAs,GaSb, InP, InAs, InSb, AlS, AlP, AlSb, PbS, PbSe, Ge and Si and ternaryand quaternary mixtures thereof.

In a preferred embodiment, the surface of the semiconductor nanocrystalis modified to enhance the efficiency of the emissions, prior to surfacemodification with the multiply amphipathic dispersant, by adding anovercoating layer or shell to the semiconductor nanocrystal. The shellis preferred because at the surface of the semiconductor nanocrystal,surface defects can result in traps for electrons or holes that degradethe electrical and optical properties of the semiconductor nanocrystal.An insulating layer at the surface of the semiconductor nanocrystalprovides an atomically abrupt jump in the chemical potential at theinterface that eliminates energy states that can serve as traps for theelectrons and holes. This results in higher efficiency in theluminescent process.

Suitable materials for the shell include semiconductor materials havinga higher bandgap energy than the semiconductor nanocrystal core, Inaddition to having a bandgap energy greater than the semiconductornanocrystal core, suitable materials for the shell should have goodconduction and valence band offset with respect to the coresemiconductor nanocrystal. Thus, the conduction band is desirably higherand the valence band is desirably lower than those of the coresemiconductor nanocrystal. For semiconductor nanocrystal cores that emitenergy in the visible (e.g., CdS, CdSe, CdTe, ZnSe, ZnTe, GaP, GaAs) ornear IR (e.g., InP, InAs, InSb, PbS, PbSe), a material that has abandgap energy in the ultraviolet regions may be used. Exemplarymaterials include ZnS, GaN, and magnesium chalcogenides, e.g., MgS,MgSe, and MgTe. For a semiconductor nanocrystal core that emits in thenear IR, materials having a bandgap energy in the visible, such as CdSor CdSe, may also be used. The preparation of a coated semiconductornanocrystal may be found in, e.g., Dabbousi et al. (1997) J. Phys. Chem.B 101:9463, Hines et al. (1996) J. Phys. Chem. 100: 468-471, Peng et al.(1997) J. Am, Chem. Soc. 119:7019-7029, and Kuno et al. (1997) J. Phys.Chem. 106:9869.

The nanoparticles of the invention may also be metallic. Such particlesare useful, for example, in surface enhanced Raman scattering (SERS),which employs nanometer-size particles onto which Raman active moieties(e.g., a dye or pigment, or a functional group exhibiting acharacteristic Raman spectrum) are adsorbed or attached. Metallicnanoparticles may be comprised of any metal or metallic alloy orcomposite, although for use in SERS, a SERS active metal is used, e.g.,silver, gold, copper, lithium, aluminum, platinum, palladium, or thelike. In addition, the particles can be in a core-shell configuration,e.g., a gold core may be encased in a silver shell; see, e.g., Freemanet al. (1996) J. Phys. Chem. 100:718-724, or the particles may formsmall aggregates in solution. Kneipp et al. (1998) Applied Spectroscopy52:1493.

Generally, and as alluded to above, the unmodified nanoparticles--andthus the inner core of the present surface-modified nanoparticles--alsocomprise a hydrophobic coating on the semiconductive or metallicmaterial resulting from solvents and/or surfactants used in nanoparticlemanufacture. For example, semiconductive nanoparticles, as manufactured,will typically have a water-insoluble organic coating that has affinityfor the semiconductive material, the coating comprised of a passivatinglayer resulting from use of a coordinating solvent such ashexyldecylamine or a trialkyl phosphine or trialkyl phosphine oxide,e.g., trioctylphosphine oxide (TOPO), trioctylphosphine (TOP), ortributylphosphine (TBP). Hydrophobic surfactants typically used in themanufacture of metallic nanoparticles and forming a coating thereoninclude, by way of example, octanethiol, dodecanethiol, dodecylamine,and tetraoctylammonium bromide. Metallic inner cores will typically havea surfactant coating that has affinity for the metallic material, thecoating similarly deriving from surfactant compounds used in themanufacture of metallic nanoparticles. The surfactant coating iscomprised of a hydrophobic surfactant.

III. The Dispersant:

The dispersant used to modify the hydrophobic surface of thenanoparticles is a multiply amphipathic dispersant, i.e., a compoundhaving two or more hydrophobic regions and two or more hydrophilicregions. In a preferred embodiment, the multiply amphipathic dispersantis polymeric, and may be composed of either a linear or branchedpolymer, whether naturally occurring, chemically modified, or chemicallysynthesized. Structurally, polymers are classified as either linear orbranched wherein the term “branched” generally means that the individualmolecular units (i.e., monomer units) of the branches are discrete fromthe polymer backbone, and may or may not have the same chemicalconstitution as the polymer backbone.

As will be appreciated by those of ordinary skill in the art, thesimplest branched polymers are the “comb branched” polymers wherein alinear backbone bears one or more essentially linear pendant sidechains. This simple form of branching may be regular or irregular (inthe latter case, the branches are distributed in non-uniform or randomfashion on the polymer backbone). An example of regular comb branchingis a comb branched polystyrene as described by Altores et al. (1965) J.Polymer Sci., Part A 3:4131-4151, and an example of irregular combbranching is illustrated by the graft copolymers described by Sorensonet al. in Preparative Methods of Polymer Chemistry, 2nd Ed.,Interscience Publishers, pp. 213-214 (1968).

The amphipathic dispersant may also be a branched polymer in the form ofa cross-linked or network polymer, i.e., a polymeric structure whereinindividual polymer chains or branches are connected through the use ofbifunctional compounds; e.g., acrylic acid monomer units bridged orcrosslinked with a diamine linker. In this type of branching, many ofthe individual branches are not linear in that each branch may itselfcontain side chains pendant from a linear chain and it is not possibleto differentiate between the backbone and the branches. Moreimportantly, in network branching, each polymer macromolecule (backbone)is cross-linked at two or more sites to other polymer macromolecules.Also, the chemical constitution of the cross-linkages may vary from thatof the polymer macromolecules. In this cross-linked or network branchedpolymer, the various branches or cross-linkages may be structurallysimilar (termed “regularly” cross-linked) or they may be structurallydissimilar (termed “irregularly” cross-linked).

The amphipathic dispersant may also have other structuralconfigurations, e.g., it may be a star/comb-branched type polymer, asdescribed in U.S. Pat. Nos. 4,599,400 and 4,690,985, or a rod-shapeddendrimer as disclosed in U.S. Pat. No. 4,694,064.

Particularly preferred amphipathic dispersants herein are hyperbranched(containing two or more generations of branching) or dendrimeric. Incontrast to hyperbranched polymers, dendrimers are regularly branchedmacromolecules with a branch point at each repeat unit. Also,hyperbranched polymers are obtained via a polymerization reaction, whilemost regular dendrimers are obtained by a series of stepwise couplingand activation steps. Examples of dendrimers include the polyamidoamine(PAMAM) Starburst® dendrimers of Tomalia et al. (1985) Polym. J. 17:117,the convergent dendrimers of Hawker et al. (1990) J. Am. Chem. Soc.112:7638, and diaminobutane dendrimers, described in Tomalia et al.(1990) Angew. Chem., Int. Ed. Engl. 29:135-175. With both hyperbranchedpolymers and dendrimers, however, the increased number of hydrophobicand hydrophilic regions amplifies the effect of the dispersant on thenanoparticle core, with respect to both affinity for the nanoparticlesurface (i.e., affinity of the hydrophobic regions of the dispersant forthe hydrophobic surface of the nanoparticle) and water dispersibility(as a result of the increased number of hydrophilic regions orsegments).

The hydrophilic regions represent approximately 30 wt. % to 75 wt. % ofthe amphipathic dispersant, and are comprised of at least one monomerunit containing an ionizable or polar moiety, preferably an ionizablemoiety such as a carboxylic acid, sulfonic acid, phosphonic acid oramine substituent. Examples of hydrophilic monomer units include, butare not limited to:

water-soluble ethylenically unsaturated C₃-C₆ carboxylic acids, such asacrylic acid, alkyl acrylic acids (particularly methacrylic acid),itaconic acid, maleic acid, fumaric acid,acrylamidomethyl-propanesulfonic acid, vinyl sulfonic acid, vinylphosphonic acid, vinyllactic acid, and styrene sulfonic acid;

allylamine and allylamine salts formed with an inorganic acid, e.g.,hydrochloric acid;

di-C₁-C₃-alkylamino-C₂-C₆-alkyl acrylates and methacrylates suchdimethylaminoethyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl acrylate, diethylaminoethyl methacrylate,dimethylaminopropyl acrylate, dimethylaminobutyl acrylate,dimethylaminoneopentyl acrylate and dimethylaminoneopentyl methacrylate;

olefinically unsaturated nitriles, such as acrylonitrile;

diolefinically unsaturated monomers, particularly diallylammoniumcompounds such as dimethyldiallylammonium chloride,dimethyldiallylammonium bromide, diethyldiallylammonium chloride,methyl-t-butyldiallylammonium methosulfate,methyl-n-propyldiallylammonium chloride, dimethyldiallylammoniumhydrogensulfate, dimethyldiallylammonium dihydrogenphosphate,di-n-butyldiallylammonium bromide, diallylpiperidinium bromide,diallylpyrrolidinium chloride and diallylmorpholinium bromide;

N-vinylpyrrolidone;

N-vinylformamide;

acrylamide and substituted acrylamides, such as N-methylolacrylamide andC₁-C₃ alkyl acrylamides, particularly methacrylamide;

N-vinylimidazole and N-vinylimidazoline; and

other monomers, typically ethylenically unsaturated monomers, preferablyvinyl monomers, substituted with at least one hydrophilic functionalitysuch as a carboxylate, a thiocarboxylate, an amide, an imide, ahydrazine, a sulfonate, a sulfoxide, a sulfone, a sulfite, a phosphate,a phosphonate, a phosphonium, an alcohol, a thiol, a nitrate, an amine,an ammonium, or an alkyl ammonium group —[NHR¹R²]⁺, wherein R¹ and R²are alkyl substituents and the group is associated with a negativelycharged anion, e.g., a halogen ion, nitrate, etc. The hydrophilicfunctionality may be directly bound to a carbon atom in the polymerbackbone, but will usually be bound through a linkage that provides somedegree of spacing between the polymer backbone and the hydrophilicfunctional group. Suitable linkages include, but are not limited to,branched or unbranched alkylene, branched or unbranched alkenylene,branched or unbranched heteroalkylene (typically alkylene containing oneor more ether or —NH— linkages) a branched or unbranchedheteroalkenylene (again, typically alkenylene containing one or moreether or —NH— linkages), arylene, heteroarylene, alkarylene, aralkylene,and the like. The linkage will typically contain 2 to 24, more typically2 to 12, carbon atoms.

The hydrophilic regions may also be composed of partially or fullyhydrolyzed poly(vinyl alcohol), poly(ethylene glycol), poly(ethyleneoxide), highly hydrated poly(alkylene oxides) such as poly(ethyleneoxide), cellulosic segments (e.g., comprised of cellulose per se orcellulose derivatives such as hydroxypropyl cellulose, hydroxyethylcellulose, hydroxypropyl methyl cellulose, methyl cellulose, ethylcellulose, cellulose acetate, and the like), and polysaccharides such aschitosan or dextran.

The hydrophobic regions represent approximately 25 wt. % to 90 wt. % ofthe amphipathic dispersant, and are comprised of at least onenon-ionizable, nonpolar monomer unit, facilitating noncovalentassociation with the hydrophobic surface of the nanoparticle. Examplesof such monomer units include, but are not limited to:

acrylates such as methacrylate, methyl methacrylate, ethyl methacrylate,butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, isodecylmethacrylate, lauryl methacrylate, phenyl methacrylate, isopropylacrylate, isobutyl acrylate and octadecylacrylate,

alkylenes such as ethylene and propylene;

C₄-C₁₂-alkyl-substituted ethyleneimine;

alkyl acrylamides wherein the alkyl group is larger than lower alkyl(particularly alkyl acrylamides wherein the alkyl group has six or morecarbon atoms, typically six to twelve carbon atoms, such ashexylacrylamide, octylacrylamide, and the like);

styrene and hydrophobically derivatized styrenes (i.e., styrenesubstituted with one or more hydrophobic substituents, e.g., C₅-C₁₂hydrocarbyl groups);

vinyl ether;

vinyl esters such as vinyl acetate; and

vinyl halides such as vinyl chloride.

The hydrophobic regions may also be composed of polychloroprene,polybutadiene, polysiloxane, polydimethylsiloxane, polyisobutylene orpolyurethane blocks, or they may be polycondensates of2-poly(hydroxyalkanoic acids) such as 2-hydroxypropanoic acid,2-hydroxybutanoic acid, 2-hydroxyisobutanoic acid, 2-hydroxyheptanoicacid, 10-hydroxydecanoic acid, 12-hydroxydodecanoic acid,12-hydroxystearic acid, 16-hydroxyhexadecanoic acid, 2-hydroxystearicacid, 2-hydroxyvaleric acid or the corresponding condensates obtainedfrom lactones, condensates of diols and dicarboxylic acids such aspolyethylene adipate, or polylactams such as polycaprolactam.

Any of the aforementioned monomer units and polymer segments can bemodified using techniques and reagents routinely used by those ofordinary skill in the art. Such modifications include, for example,routine substitutions, additions of chemical groups such as alkyl groupsand alkylene groups, hydroxylations, oxidations, and the like. Suchbranched polymers, composed of hydrophobic segments and hydrophilicsegments, are typically comprised of (1) a hydrophobic backbone withhydrophilic branches, (2) a hydrophilic backbone with hydrophobicbranches, or (3) a backbone that may be either hydrophobic orhydrophilic, and is substituted with both hydrophilic and hydrophobicbranches. Such polymers can be prepared by any suitable method readilyknown to those of ordinary skill in the art and/or described in thepertinent texts and literature. Polymers of type (1), for example, canbe prepared by copolymerization of a hydrophobic monomer with a secondmonomer that includes suitable reactive groups through which thehydrophilic side chains (branches) can be grafted to the backbone.Alternatively, type (1) polymers can be prepared by polymerizing asingle hydrophobic monomer with a suitable reactive side group, and afraction of those reactive side groups can be modifiedpost-polymerization by grafting hydrophilic side chains. Analogously,polymers of type (2) can be prepared by copolymerization of ahydrophilic monomer with a second monomer that includes suitablereactive groups through which the hydrophobic side chains (branches) canbe grafted to the backbone. Alternatively, type (2) polymers can beprepared by polymerizing a single hydrophilic monomer with a suitablereactive side group, and a fraction of those reactive side groups can bemodified post-polymerization by grafting hydrophobic side chains. Type(3) polymers can be prepared by first synthesizing a linear polymerhaving reactive sites throughout the backbone, and then graftinghydrophilic and hydrophobic side chains onto the backbone in a fashionthat may or may not be ordered.

Particularly preferred amphipathic dispersants include acrylic acid andmethacrylic acid polymers modified to include hydrophobic regions, aswell as copolymers of acrylic acid and/or methacrylic acid withhydrophobic comonomers such as alkyl acrylamides. Examples of suchpolymers are poly(acrylic acid-co-octylacrylamide), poly(acrylicacid-co-hexylacrylamide), poly(methacrylic acid-co-octylacrylamide), andpoly(methacrylic acid-co-hexylacrylamide), with poly(acrylicacid-co-octylacrylamide) most preferred. The specific methodology usedto synthesize polymers suitable as the multiply amphipathic dispersantwill depend on the particular monomer types that are employed. As willbe appreciated by those of ordinary skill in the art, suitablepolymerization techniques include step polymerization, radical chainpolymerization, emulsion polymerization, ionic chain polymerization,chain copolymerization, ring-opening polymerization, livingpolymerization, polycondensation reactions, and graft polymerization. Ina preferred embodiment, the amphipathic dispersant is formed by additionpolymerization of ethylenically unsaturated monomers. Suchpolymerization reactions are generally catalyzed using metalliccatalysts (e.g., transition metal-based metallocenes, Ziegler-Nattacatalysts, Brookhart-type catalysts, etc.) and typically involvecontacting the monomer(s), catalyst, and a catalyst activator (e.g.,methyl aluminoxane, or “MAO”) at a suitable temperature at reduced,elevated or atmospheric pressure, under an inert atmosphere, for a timeeffective to produce the desired polymer. An added solvent may, ifdesired, be employed, or the monomeric compounds may serve as solvent.The reaction may be conducted under solution or slurry conditions, in asuspension, or in the gas phase. As alluded to above, branched polymerscan be prepared using this technique by introducing reactive sites intothe polymer backbone during polymerization (e.g., by incorporating somefraction of monomer units having a pendant reactive site), followed bysynthesis or grafting of branches at the reactive sites.

In a preferred embodiment, the amphipathic dispersant is comprised of ahydrophilic backbone that has been modified to contain hydrophobicanchoring groups, i.e., hydrophobic side chains that serve to “anchor”the dispersant to the nanoparticle surface. For example, hydrophilicpolymers containing pendant carboxylic acid groups (e.g., as inpoly(acrylic acid), [—(CH₂CH(CO₂H)]_(n)—) can be readily modified tocontain a controlled number of branched or unbranched hydrophobic sidechains using methods known in the art. In one such method, the pendantcarboxylic acid groups of poly(acrylic acid) can be activated with asuitable activating agent, e.g., thionyl chloride or a carbodiimide,followed by reaction with a long chain alkylamine, e.g., a C₄-C₁₂alkylamine such as octylamine, and finally with a hydrolyzing agent suchas water. Depending on the relative quantities of the alkylamine and thehydrolyzing agent, the resulting polymer is an amphipathic polymer witha hydrophilic backbone (by virtue of the carboxylic acid groups presentafter partial hydrolysis) and hydrophobic side chains (the long chainalkyl group attached to the backbone through an amide linkage).

Within the aforementioned group of hydrophobically modified hydrophilicpolymers are hydrophobically modified peptides, preferablyhydrophobically modified synthetic polypeptides. The use of syntheticpolypeptides allows for control over a number of factors, including themonodispersity of the molecular weight of the hydrophilic backbone, thenumber and position of modifiable groups on the backbone, and theregularity of the modification, i.e., whether the hydrophobic groups arerandomly distributed throughout the polypeptide chain or present in anordered, “regular” fashion.

Suitable polypeptides are triblock (A-B-A) copolymers, for example,triblock copolymers of aspartate and norleucine, in which casepolynorleucine is preferably the central block “B.” Such a triblockcopolymer provides a region rich in hydrophobic side chains. In onealternative, the central block “B” can comprise a hydrophilic aminoacid, for example, poly(lysine), which can be modified via standardchemistries to include hydrophobic side chains. The carboxylate-richaspartate side chains (A) provide the polar, ionic groups that not onlyaid in rendering the nanocrystal water dispersible, but provide reactivesites or functionalizable moieties for further chemistry, such asconjugation to affinity molecules.

The polypeptide compositions of the present invention may also bemonofunctional in nature, e.g., polylysine or polyaspartate, diblockcopolymers (A-B) or triblock copolymers of three different amino acids(A-B-C). These compositions are also not restricted to lysine oraspartate, but may make use of any number of combinations of the knownamino acids. Generally, the hydrophobic regions of a polypeptide arecomprised of at least one hydrophobic amino acid and the hydrophilicregions are comprised of at least one hydrophilic amino acid. As will beappreciated by those of ordinary skill in the art, hydrophobic aminoacids include, for example, alanine, glycine, valine, leucine,isoleucine, norleucine, proline, phenylalanine, methionine, tryptophane,cysteine, and includes hydrophilic amino acids modified to includehydrophobic side chains, while hydrophilic amino acids include asparticacid, glutamic acid, lysine, arginine, histidine, asparagine, glutamine,serine, threonine and tyrosine.

The amphipathic dispersant generally although not necessarily has amolecular weight in the range of approximately 500 to 50,000, preferablyin the range of approximately 1000 to 10,000, more preferably in therange of approximately 1000 to 5000. The dispersant may be modified soas to contain functionalizable sites useful for covalent or noncovalentattachment to an external molecular moiety. The functionalizable sitesmay be present in addition to the ionizable groups discussed above, orthe ionizable groups may themselves serve as functionalizable sitessuitable for binding an external molecular moiety. Functionalizablesites include, for example, any of the conventional functional groupsthat are modified using simple, conventional chemical techniques, e.g.,amino groups, nitriles, carboxylic acids, esters, acid chlorides, andthe like. Preferably, although not necessarily, the functionalizablesites are spaced apart from the dispersant structure by an inert linkingmoiety, e.g., an alkylene or oxyalkylene chain, typically composed ofabout 2 to 20 carbon atoms, preferably about 4 to 10 carbon atoms, orother linking moieties such as those described above with respect to thespacer linkages that may be present linking hydrophilic functionalgroups to the polymer backbone.

IV. Preparation of the Surface-Modified Nanoparticles:

Hydrophobic nanoparticles may be rendered water dispersible by surfacemodification with the amphipathic dispersant. That is, the hydrophobicregions of the dispersant associate with the hydrophobic nanoparticlesurface, and the hydrophilic regions are externally facing and providewater dispersibility. Surface modification of the nanoparticles iscarried out as follows.

Initially, a solution of the amphipathic dispersant is prepared byadmixing the selected amphipathic dispersant with a suitable nonaqueoussolvent, preferably a nonpolar, water-immiscible solvent such asn-hexane or chloroform. Ionizable groups on the dispersant, if present,are then converted to salt form by treatment with an appropriate acid orbase, which serves as an ionizing agent. For ionizable acidic groups,suitable bases are generally inorganic bases, e.g., ammonium hydroxidesor hydroxides of alkali metals (e.g., sodium or potassium) or alkalineearth metals (e.g., magnesium or calcium). The hydrophobic nanoparticlesare dispersed in the same solvent, either before or after theaforementioned ionization step. Typically, however, the nanoparticlesare added after ionization, preferably dropwise, to a stirring solutionof the ionized dispersant. Alternatively, the nanoparticles may bedispersed in the solvent at the outset, and the dispersant addedthereto. As another alternative, two separate solutions may be preparedand mixed, with one solution containing the dispersant and the othersolution containing the nanoparticles, with both solutions preferablycontaining the same solvent. In all cases, after preparation of thenanoparticle-dispersant-solvent admixture, the admixture is preferablystirred for several minutes to ensure complete mixing of the components.

In the next step of the process, the admixture of nanoparticles,dispersant and solvent is subjected to conditions effective to result inabsorption of the dispersant by the nanoparticles. For example, theadmixture may be heated or placed under vacuum to remove the solvent,such a drying process resulting in dispersant-coated nanoparticles.Alternatively, the conditions may involve changing the polarity of thesolvent and/or changing the ionic state of the polymer.

Next, the dispersant-coated nanoparticles are transferred to an aqueousmedium such as water, using solvent exchange (if the dispersant-coatednanoparticles are not previously dried) or addition of water or anaqueous buffer (if the dispersant-coated nanoparticles are previouslydried). The aqueous buffer, if one is used, should be effective tofacilitate dispersion of the nanoparticles in the aqueous medium. Thewater dispersion is then filtered to remove any large micellarstructures formed by excess dispersant in solution that is notassociated with the particles. These materials may then be used in anyapplications requiring aqueous-based sols of nanocrystals. Prior tousing these particles one may further increase the stability of theamphipathic coating by chemically crosslinking the individual polymerchains of the dispersant coating such that each polymer has a potentialmultiplicity of chemical bonds to other polymer chains on the particle.One of ordinary skill in the art would recognize that the crosslinkerused may be tailored to match the properties of the dispersant coating.For example, a diamine could be used to crosslink a dispersant coatingcontaining carboxylic acids. Of particular utility are crosslinkers thatcarry charges or other groups capable of stabilizing the dispersedcolloids as described herein. A diamino carboxylate or sulfonate and adiamino polyethylene glycol crosslinkers are especially useful. Asimilar chemistry would apply for crosslinkers having multiple aminemoieties, such as dendrimers, modified dendrimers, and the like.

The amount of amphipathic dispersant per unit mass of the “inner core”(i.e., per unit mass of the original, unmodified nanoparticle) in theresulting dispersant-coated nanoparticles is proportional to the sizeand surface area of the nanoparticles. Generally, the number ratio ofthe dispersant to the inner core will be in the range of approximately50:1 to approximately 5000:1. The ratio will be closer to 50:1 forsmaller nanoparticles, i.e., nanoparticles less than about 5 nm indiameter (e.g., green CdSe quantum dots), and will be closer to 5000:1for larger nanoparticles, i.e., nanoparticles about 5 nm to 10 nm indiameter (e.g., red CdSe quantum dots).

V. Nanoparticle Conjugates and Associated Compositions:

The invention additionally relates to conjugates of the presentsurface-modified semiconductive nanoparticles and compositionscomprising those conjugates in association with a target analyte.

That is, the surface-modified semiconductive nanoparticles of theinvention may be conjugated to an affinity molecule that serves as thefirst member of a binding pair. Generally, although not necessarily, itis the amphipathic dispersant on the nanoparticle surface that providesthe means for linkage to the affinity molecule. As noted previously,ionizable groups present within the hydrophilic regions of theamphipathic dispersant may provide the means for linkage to the affinitymolecule, and/or other functional groups present within or introducedinto the dispersant molecule may provide the means for linkage to theaffinity molecule. The linkage will generally be covalent, and suitablelinkers are discussed in Section III, above. Suitable methods ofconjugating molecules and molecular segments to affinity molecules aredescribed, for example, in Hermanson, Bioconjugate Techniques (AcademicPress, NY, 1996).

Such semiconductive nanoparticle “conjugates,” by virtue of the affinitymolecule, can be used to detect the presence and/or quantity ofbiological and chemical compounds, interactions in biological systems,biological processes, alterations in biological processes, oralterations in the structure of biological compounds. That is, theaffinity molecule, when linked to the semiconductive nanoparticle, caninteract with a biological target that serves as the second member ofthe binding pair, in order to detect biological processes or reactions,or to alter biological molecules or processes. Preferably, theinteraction of the affinity molecule and the biological target involvesspecific binding, and can involve covalent, noncovalent, hydrophobic,hydrophilic, electrostatic, van der Waal's, or magnetic interaction.Preferably, the affinity molecule physically interacts with thebiological target.

The affinity molecule associated with the semiconductive nanoparticlescan be naturally occurring or chemically synthesized, and can beselected to have a desired physical, chemical, or biological property.Such properties include, but are not limited to, covalent andnoncovalent association with proteins, nucleic acids, signalingmolecules, prokaryotic or eukaryotic cells, viruses, subcellularorganelles and any other biological compounds. Other properties of suchmolecules include, but are not limited to, the ability to affect abiological process (e.g. cell cycle, blood coagulation, cell death,transcription, translation, signal transduction, DNA damage or cleavage,production of radicals, scavenging radicals, etc.), and the ability toalter the structure of a biological compound (e.g. crosslinking,proteolytic cleavage, radical damage, etc

In a preferred embodiment, the nanoparticle conjugate is comprised of asemiconductive nanoparticle that emits light at a tunable wavelength andis associated with a nucleic acid. The association can be direct orindirect. The nucleic acid can be any ribonucleic acid, deoxyribonucleicacid, dideoxyribonucleic acid, or any derivatives and combinationsthereof. The nucleic acid can also be oligonucleotides of any length.The oligonucleotides can be single-stranded, double-stranded,triple-stranded or higher order configurations (e.g. Holliday junctions,circular single-stranded DNA, circular double-stranded DNA, DNA cubes,(see Seeman (1998) Ann. Rev. Biophys. Biomol. Struct. 27:225-248). Amongthe preferred uses of the present compositions and methods are detectingand/or quantitating nucleic acids as follows: (a) viral nucleic acids;(b) bacterial nucleic acids; and (c) numerous human sequences ofinterest, e.g. single nucleotide polymorphisms. Without limiting thescope of the present invention, nanoparticle conjugates can comprisenanocrystals associated with individual nucleotides, deoxynucleotides,dideoxynucleotides or any derivatives and combinations thereof and usedin DNA polymerization reactions such as DNA sequencing, reversetranscription of RNA into DNA, and polymerase chain reactions (PCR).Nucleotides also include monophosphate, diphosphate and triphosphatesand cyclic derivatives such as cyclic adenine monophosphate (cAMP).Other uses of nanoparticles conjugated to nucleic acids includedfluorescence in situ hybridization (FISH). In this preferred embodiment,nanocrystals are conjugated to oligonucleotides designed to hybridize toa specific sequence in vivo. Upon hybridization, the fluorescentnanocrystal tags are used to visualize the location of the desired DNAsequence in a cell. For example, the cellular location of a gene whoseDNA sequence is partially or completely known can be determined usingFISH. Any DNA or RNA whose sequence is partially or completely known canbe visually targeted using FISH. For example without limiting the scopeof the present invention, messenger RNA (mRNA), DNA telomeres, otherhighly repeated DNA sequences, and other non-coding DNA sequencing canbe targeted by FISH.

The nanoparticle conjugate may also comprise a surface-modifiedsemiconductive nanoparticle as provided herein in association with amolecule or reagent for detection of biological compounds such asenzymes, enzyme substrates, enzyme inhibitors, cellular organelles,lipids, phospholipids, fatty acids, sterols, cell membranes, moleculesinvolved in signal transduction, receptors and ion channels. Theconjugate also can be used to detect cell morphology and fluid flow;cell viability, proliferation and function; endocytosis and exocytosis(Betz et al. (1996) Curr. Opin. Neurobiol. 6(3):365-71); and reactiveoxygen species (e.g., superoxide, nitric oxide, hydroxyl radicals,oxygen radicals). In addition, the conjugate can be used to detecthydrophobic or hydrophilic regions of biological systems.

Conjugates of semiconductive nanocrystals also find utility in numerousother biological and non-biological applications where luminescentmarkers, particularly fluorescent markers, are typically used. See, forexample, Haugland, R.P. Handbook of Fluorescent Probes and ResearchChemicals (Molecular Probes, Eugene, Oreg. Sixth Ed. 1996; Website,www.probes.com.). Examples of areas in which the luminescentnanoparticle conjugates of the invention are useful include, withoutlimitation, fluorescence immunocytochemistry, fluorescence microscopy,DNA sequence analysis, fluorescence in situ hybridization (FISH),fluorescence resonance energy transfer (FRET), flow cytometry(Fluorescence Activated Cell Sorter; FACS) and diagnostic assays forbiological systems. For further discussion concerning the utility ofnanocrystal conjugates in the aforementioned areas, see InternationalPatent Publication No. WO 00/17642 to Bawendi et al.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples that follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

The following examples are intended to provide those of ordinary skillin the art with a complete disclosure and description of how to make anduse the novel compositions of the invention. Efforts have been made toensure accuracy with respect to numbers used (e.g., amounts,temperatures, etc), but some experimental error and deviation should, ofcourse, be allowed for. Unless indicated otherwise, parts are parts byweight, temperatures are in degrees centigrade, and pressure is at ornear atmospheric.

The practice of the present invention will employ, unless otherwiseindicated, conventional techniques of synthetic organic chemistry,biochemistry, molecular biology, and the like, which are within theskill of the art. Such techniques are explained fully in the literature.See, e.g., Sambrook, Fritsch & Maniatis, Molecular Cloning: A LaboratoryManual, Second Edition (1989); Oligonucleotide Synthesis (M.J. Gait,ed., 1984); Nucleic Acid Hybridization (B. D. Haines & S J. Higgins,eds., 1984); Methods in Enzymology (Academic Press, Inc.); Kirk-Othmer'sEncyclopedia of Chemical Technology; and House's Modern SyntheticReactions. All patents, patent applications, patent publications,journal articles and other references cited herein are incorporated byreference in their entireties.

EXAMPLE 1 Synthesis of Hydrophobically Modified Hydrophilic Polymers:

A modified polyacrylic acid was prepared by diluting 100 g [0.48 molCOONa] of poly(acrylic acid, sodium salt) (obtained from Aldrich,molecular weight 1200) was diluted two-fold in water and acidified in a1.0 L round bottom flask with 150 ml (1.9 mol) of concentrated HCl. Theacidified polymer solution was concentrated to dryness on a rotaryevaporator (100 mbar, 80° C.). The dry polymer was evacuated for 12hours at <10 mbar to ensure water removal. A stirbar and 47.0 g (0.24mol) of 1-[3-(dimethyl-amino)-propyl]-ethylcarbodiimide hydrochloride(EDC-Aldrich 98%) were added to the flask, then the flask was sealed andpurged with N₂, and fit with a balloon. 500 ml of anhydrousN-N,dimethylformamide (Aldrich) was transferred under positive pressurethrough a cannula to this mixture; and the flask was swirled gently todissolve the solids. 32 ml (0.19 mol) of octylamine was transferreddropwise under positive pressure through a cannula from a sealedoven-dried graduated cylinder into the stirring polymer/EDC solution,and the stirring continued for 12 hours. This solution was concentratedto <100 ml on a rotary evaporator (30 mbar, 80° C.), and the polymer wasprecipitated by addition of 200 ml di-H₂O to the cooled concentrate,which produced a gummy white material. This material was separated fromthe supernatant and triturated with 100 ml di-H₂O three more times. Theproduct was dissolved into 400 ml ethyl acetate (Aldrich) with gentleheating, and basified with 200 ml di-H₂O and 100 gN-N-N-N-tetramethylammonium hydroxide pentahydrate (0.55 mo) for 12hours. The aqueous layer was removed and precipitated to a gummy whiteproduct with 400 ml of 1.27 M HCl. The product was decanted andtriturated with 100 ml of di-H₂O twice more, after which the aqueouswashings were back-extracted into 6×100 ml portions of ethyl acetate.These ethyl acetate solutions were added to the product flask, andconcentrated to dryness (100 mbar, 60° C.). The crude polymer wasdissolved in 300 ml of methanol and purified in two aliquots over LH-20(Amersham-Pharmacia-5.5 cm×60 cm column) at a 3 ml/minute flow rate.Fractions were tested by NMR for purity, and the pure fractions werepooled, while the impure fractions were re-purified on the LH-20 column.After pooling all of the pure fractions, the polymer solution wasconcentrated by rotary evaporation to dryness, and evacuated for 12hours at <10 mbar. The product was a white powder (25.5 g, 45% oftheoretical yield), which showed broad NMR peaks in CD₃OD [δ=3.1 b(9.4), 2.3 b (9.7), 1.9 1.7 1.5 1.3 b (63.3) 0.9 bt (11.3)], and clearIR signal for both carboxylic acid (1712 cm⁻¹) and amide groups (1626cm⁻¹, 1544 cm⁻¹).

EXAMPLE 2 Preparation of Surface-Modified Nanocrystals:

Twenty milliliters of 3-5 μM (3-5 nmoles) of TOPO/TOP coated CdSe/ZnSnanocrystals (see, Murray et al. (1993) J. Am. Chem. Soc. 115:8706) wereprecipitated with 20 milliliters of methanol. The flocculate wascentrifuged at 3000×g for 3 minutes to form a pellet of thenanocrystals. The supernatant was thereafter removed and 20 millilitersof methanol was again added to the particles. The particles werevortexed to loosely disperse the flocculate throughout the methanol. Theflocculate was centrifuged an additional time to form a pellet of thenanocrystals. This precipitation/centrifugation step was repeated anadditional time to remove any excess reactants remaining from thenanocrystal synthesis. Twenty milliliters of chloroform were added tothe nanocrystal pellet to yield a freely dispersed sol.

300 milligrams of hydrophobically modified poly(acrylic acid) wasdissolved in 20 ml of chloroform. Tetrabutylammonium hydroxide (1.0 M inmethanol) was added to the polymer solution to raise the solution to pH10 (pH was measured by spotting a small aliquot of the chloroformsolution on pH paper, evaporating the solvent and thereafter wetting thepH paper with distilled water). Thereafter the polymer solution wasadded to 20 ml of chloroform in a 250 ml round bottom flask equippedwith a stir bar. The solution was stirred for 1 minute to ensurecomplete admixture of the polymer solution. With continued stirring thewashed nanocrystal dispersion described above was added dropwise to thepolymer solution. The dispersion was then stirred for two minutes toensure complete mixing of the components and thereafter the chloroformwas removed in vacuo with low heat to yield a thin film of theparticle-polymer complex on the wall of the flask. Twenty milliliters ofdistilled water were added to the flask and swirled along the walls ofthe flask to aid in dispersing the particles in the aqueous medium. Thedispersion was then allowed to stir overnight at room temperature. Atthis point the nanocrystals are freely dispersed in the aqueous medium,possess pendant chemical functionalities and may therefore be linked toaffinity molecules of interest using methods well known in the art forbiolabeling experiments. In addition, the fact that the nanocrystals nowhave a highly charged surface means they can be readily utilized inpolyelectrolyte layering experiments for the formation of thin films andcomposite materials.

EXAMPLE 3 Preparation of Nanocrystal Conjugates:

Functional and specific biological labels have been made with materialsof the present invention as follows: The polymer-stabilized particlesfrom Example 1 were purified away from excess (non-absorbed) polymer andtetrabutylammonium hydroxide via tangential flow diafiltration using a100 K polyethersulfone membrane against one liter of distilled water andone liter of 50 mM morpholinoethanesulfonic acid buffer, pH 5.9. Thepurified dispersion was concentrated to 20 milliliters and 10milliliters of this nanocrystal dispersion were activated with 79 μmoles(15 mg) 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride(EDC) and 158 μmoles (34 mg) N-hydroxysulfosuccinimide for 30 minutes atroom temperature. The particle dispersion was then buffer exchanged topH 8.0 via diafiltration against 50 mM phosphate buffer, pH 8.0. Whenthe particle dispersion reached pH 8.0, streptavidin was added to theparticles at a 5:1 protein:particle ratio (175 nmoles, 10.5 mg) and thereaction mixture was incubated overnight at room temperature withstirring. After overnight incubation the conjugated particles wereseparated from excess, unreacted protein via tangential flowdiafiltration using a 100,000 MW polyethersulfone membrane against 2liters of phosphate buffer, 50mM, pH 7.0.

At this point these materials can be stored in any number of biologicalbuffers and used as fluorescent biological labels to detectbiotin-labeled analytes of interest. Although streptavidin was used hereas an example, the simplicity and generality of the above couplingchemistries can be efficiently extended to forming functional conjugateswith any number of biological molecules of interest, such as antibodies,peptides, and oligonucleotides, for example.

EXAMPLE 4

Crosslinking of Polymer Stabilized Nanocrystals with a Dendrimer:

Ten milliliters of nanocrystals at 3.5 μM, stabilized as described inExample 2, were purified by tangential flow filtration, as described inExample 3, against 1 liter of distilled water to remove excess polymer.The nanocrystals were concentrated to 10 milliliters and the pH of theaqueous dispersion was decreased to pH 6.5 with 50 μl additions of 0.1 MHCl. 67 milligrams (315 moles) EDC were added to the stirringnanocrystal dispersion. The reaction was allowed to proceed for 10minutes before 1 milliliter of 0.5M borate buffer (pH 8.5) containing3.94 moles of the crosslinking reagent Starburst® (PAMAM) Dendrimer,Generation 0, were added to the reaction mixture. The reaction mixturewas stirred for 2 hours at room temperature and then transferred to a50,000 molecular weight cut-off polyethersulfone dialysis bag. Dialysiswas performed for 24 hours against 2 changes of 4 liters of water.

EXAMPLE 5

Crosslinking of Polymer Stabilized Nanocrystals with a DiaminoCrosslinker:

Ten milliliters of nanocrystals at 3.5 μM, stabilized as described inExample 2, were purified by tangential flow filtration, as described inExample 3, against 1 liter of distilled water to remove excess polymer.The nanocrystals were concentrated to 10 milliliters and the pH of theaqueous dispersion was decreased to pH 6.5 with 50 μl additions of 0.1 MHCl. 67 milligrams (315 μmoles) EDC were added to the stirringnanocrystal dispersion. The reaction was allowed to proceed for 10minutes before 1 milliliter of 0.5M borate buffer (pH 8.5) containing3.94 μmoles of the crosslinking reagent lysine (a diamino carboxylicacid) were added to the reaction mixture. The reaction mixture wasstirred for 2 hours at room temperature and then transferred to a 50,000molecular weight cut-off polyethersulfone dialysis bag. Dialysis wasperformed for 24 hours against 2 changes of 4 liters of water.

EXAMPLE 6

Preparation of Surface Modified Nanocrystals with Polypeptides:

A triblock polypeptide comprised of(Aspartate)₄-(Norleucine)₈-(Aspartate)₄ has been used to stabilizehydrophobic nanocrystals in water by the following method: Fivemilliliters of a 3.5 μM nanocrystal solution were washed as described inExample 1 and redispersed in 5 milliliters of chloroform. 75 milligramsof an (Aspartate)₄-(Norleucine)₈-(Aspartate)₄ triblock polypeptide weredissolved in 5 milliliters of a 50:50 mixture of chloroform:methanol andthe pH of the polypeptide solution was raised to 10 with aliquots oftetrabutylammonium hydroxide (1.0 M in methanol). This polypeptidesolution was then added to 5 milliliters of chloroform in a 50milliliter round bottom flask. The solution was allowed to stir for 1minute to ensure complete mixing. The washed nanocrystal dispersion inchloroform was then added dropwise to the stirring polypeptide solutionand the entire mixture was allowed to stir for an additional 2 minutesbefore all the solvent was removed in vacuo with low heat (40 degreesCelsius) to yield a thin film of the particle-polymer complex on thewall of the flask. Five milliliters of distilled water were then addedto the flask and swirled in order to aid in dispersing the nanocrystalsfully in the aqueous medium. As with the nanocrystals stabilized inExample 1, these polypeptide stabilized nanocrystals can be efficientlypurified away from excess polypeptide by dialysis, tangential flowfiltration, or various forms of chromatography known to those skilled inthe art.

1-9. (canceled)
 10. A population of water-dispersible nanoparticles,comprising: a plurality of hydrophobic nanoparticles, wherein eachnanoparticle is provided with an outer layer of an amphipathicdispersant, the amphipathic dispersant comprising a polymer having ahydrophobic backbone and a plurality of alkyl acrylamide side chains,wherein the alkyl group of each acrylamide side chain has more than fourcarbon atoms, and wherein the amphipathic dispersant renders thenanoparticle water-dispersible.
 11. The population of claim 10, whereinthe alkyl group of each acrylamide side chain has six or more carbonatoms.
 12. The population of claim 10, wherein the alkyl group of eachacrylamide side chain has 6 to 12 carbon atoms.
 13. The population ofclaim 10, wherein the alkyl group of each acrylamide side chain haseight or more carbon atoms.
 14. The population of claim 10, wherein theplurality of alkyl acrylamide side chains comprises octylacrylamide. 15.The population of claim 10, wherein the polymer further compriseshydrophilic side chains.
 16. The population of claim 10, wherein thepolymer further comprises ionizable groups.
 17. The population of claim10, wherein the polymer further comprises a group selected from amine,carboxylic acid, sulfonic acid, and phosphonic acid.
 18. The populationof claim 10, wherein the polymer comprises at least one ethylenicallyunsaturated C₃-C₆ carboxylic acid monomer residue.
 19. The population ofclaim 10, wherein the polymer comprises an acrylic acid or methacrylicacid monomer residue.
 20. The population of claim 10, wherein thepolymer is poly(acrylic acid-co-octylacrylamide), poly(acrylicacid-co-hexylacrylamide), poly(methacrylic acid-co-octylacrylamide), orpoly(methacrylic acid-co-hexylacrylamide).
 21. The population of claim10, wherein the nanoparticle is surrounded with the outer layer ofamphipathic dispersant.
 22. The population of claim 10, wherein two ormore nanoparticles are surrounded with the outer layer of amphipathicdispersant.
 23. The population of claim 10, wherein the amphipathicdispersant is crosslinked.
 24. The population of claim 10, whereinplurality of hydrophobic nanoparticles comprises semiconductor ormetallic nanoparticles.
 25. The population of claim 10, wherein theplurality of hydrophobic nanoparticles comprises semiconductornanocrystals.
 26. The population of claim 10, wherein the plurality ofwater-dispersible nanoparticles is dispersed in an aqueous medium.
 27. Apopulation of nanoparticle conjugates, comprising: the population ofwater-dispersible nanoparticles of claim 10, wherein each nanoparticleis associated with an affinity molecule.
 28. The population ofnanoparticle conjugates of claim 27, wherein the affinity molecule isselected from the group consisting of proteins, nucleic acids,polysaccharides, and molecules having a molecular weight of less thanabout 1500 grams/mol.
 29. The population of nanoparticle conjugates ofclaim 27, wherein the affinity molecule is selected from the groupconsisting of an antigen, hapten, antibody or fragment thereof, biotin,streptavidin, hormone, hormone binding protein, enzyme, enzymeinhibitor, receptor agonist, and receptor antagonist.
 30. An aqueousdispersion of nanoparticles, comprising: the population ofwater-dispersible nanoparticles of claim 10; and an aqueous medium,wherein the nanoparticles in the population are dispersed in the aqueousmedium and exhibit luminescence when excited by an appropriate energysource.